Are Black Holes From the Early Universe Still Around Today?

In a wildly speculative and fun new episode of the PBS series Space Time, writer and host Matt O'Dowd takes us on a journey to explore primordial black holes—theoretical black holes that formed not from the collapse of supermassive stars, the way regular black holes come to be, but from the incredibly high density of the universe immediately following the Big Bang.

Did these primordial black holes, or PBHs, ever exist? And if so, are they still around today?

Theoretical astrophysics models of the Big Bang and the universe itself suggest that if a PBH had enough mass when it formed, it could still exist today. Roughly speaking, any PBHs less than a billion metric tons—about the mass of a small asteroid—would have decayed from Hawking radiation by now and faded out of existence. But PBHs with a mass greater than a billion metric tons could still be out there.

In fact, they could be everywhere. There are even theories that suggest dark matter, which makes up 80 percent of the matter in the observable universe, is in fact billions of primordial black holes. If this were the case—that incredibly dense punctures or warpings of spacetime account for the vast majority of the universe's mass—then we would expect to see the warping of light, or microlensing, caused by the PBHs as they travel all around the cosmos. Because we don't see this microlensing very often, there is a narrow range of masses that remaining PBHs could have if they are, in fact, dark matter—making this an unlikely possibility.

That doesn't mean PBHs aren't out there in smaller numbers, though. After all, these are black holes we are talking about, the densest objects in the known universe. If a PBH had a mass of about a billion tons—the minimum for it to not have decayed—then today it would be the size of an individual proton. Such a black hole could pass right through the planet, to use O'Dowd's words, "as if the Earth were made of air."

If there are tiny, super-dense black holes passing through our planet regularly, the best chance to find them might be trace signs of Hawking radiation left behind in the rock. So to discover if PBHs still exist today, or if they indeed ever existed, perhaps we need to look below us in the geologic record rather than to the heavens above.

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